Part XII: Fusion

In Part XII: Fusion, we look at the final energy source in this series, with Conclusions coming tomorrow with all that we have learned:
Fusion is the fundamental energy source of the universe. It is the process that powers the sun and the stars. In a fusion reaction, large amounts of energy are released when the nuclei of two light atoms (deuterium and tritium) fuse together to form a heavier one, helium. Tapping into this energy source offers the prospect of a long-term, safe, environmentally friendly option to meet the energy needs of a growing world population.
Fusion is a particularly attractive energy solution as it uses a fuel that is abundant and available everywhere. The primary fuels used in fusion are deuterium and lithium. Deuterium is a hydrogen isotope, which can be readily extracted from water (there is around 30g of deuterium in every cubic metre of water), and lithium is an abundant light metal from which tritium can be generated inside the reactor.
In hydrogen atoms the centre, or nucleus, contains only one proton. In deuterium the nucleus contains a proton and one neutron, while for tritium there are two neutrons with the proton. The fusion of one deuterium nucleus with a tritium nucleus makes a new nucleus of the element helium (also known as an alpha particle), a neutron and energy – lots of it! The extra neutron can be used to generate more tritium fuel from lithium. One gram of fusion fuel could generate 100 000 kilowatt hours of electricity – to supply the equivalent power you would need to burn eight tons of coal!

Fusion reactions occur at high temperatures when the nuclei collide with sufficient energy to overcome the natural repulsive forces of their electrical charges. They occur naturally in the sun at temperatures of 10 - 15 million ºC, producing the energy that sustains life on earth. However, in the sun the fusion fuel is heated and compressed by massive gravitational forces. On earth we cannot use gravity, so the challenge for fusion researchers is to compensate by heating a lower-density plasma to a higher temperature (about 100 million ºC, or 10 times hotter than the core of the sun) with excellent thermal insulation to initiate self-sustaining fusion reactions. 100 million ºC is well above the temperature at which a gas is completely ionised and becomes a plasma, the fourth state of matter. In an ionised plasma the positively-charged nuclei and negatively-charged electrons of atoms are separated and move about freely like molecules in a gas. More than 99% of our universe exists as plasma but most of it at much lower temperatures!
To reach fusion conditions in the plasma, powerful heating is necessary and heat loss must be kept to a minimum by keeping the hot plasma thermally insulated from the reactor walls – a process known as confinement. This is a difficult task, both in terms of understanding the complex physical processes involved and developing the sophisticated technologies required to control them. Two different technologies have been developed in fusion research: magnetic confinement and inertial confinement.

Magnetic confinement uses strong magnetic fields to provide the thermal insulation of the plasma and allows the possibility of steady state operation, while interial confinement uses high-power lasers or ion beams to heat and compress minuscule pellets of fuel.

A brief history of fusion:
The first clues to how the stars function were revealed in Einstein’s deceptively simple equation E = mc2 derived in 1905 as a consequence of his special theory of relativity.
This famous equation predicted that a tiny amount of mass could, in principle, be converted into a tremendous amount of energy. Einstein’s relation generalised and extended the previous 19th century law on conservation of energy established by von Helmholtz and Mayer to include the conversion of mass into energy.
What was the connection between Einstein's equation and the energy source of the Sun? The answer was not obvious. Astronomers did their part by defining the constraints that observations of stars imposed on possible explanations for the generation of stellar energy. In 1919, Henry Norris Russell, a leading theoretical astronomer in the United States, summarised concisely the hints on the nature of the stellar energy source. Russell stressed that the most important clue was the high temperature in the interior of stars.
Francis William Aston discovered the key experimental piece of the puzzle in 1920. He made precise measurements of the masses of many different atoms, among them hydrogen and helium, and found that four hydrogen nuclei were heavier than a helium nucleus.
The importance of Aston's measurements was recognised immediately by Sir Arthur Eddington, the British astrophysicist. Eddington argued in his 1920 presidential address to the British Association for the Advancement of Science that Aston's measurement of the mass difference between four atoms of hydrogen and a helium atom meant that the sun could shine by converting hydrogen atoms into helium. This burning of hydrogen into helium would (according to E=mc2) release about 0.7% of the mass equivalent of the energy. In principle, this would allow the sun to shine for about 100 billion years.
In 1939, Hans Bethe described a quantitative theory explaining the fusion generation of energy in the stars (including our sun). The results of his calculations presented in a paper entitled "Energy Production in Stars,'' won him the Nobel prize for Physics in 1968.
With the general theory for fusion reactions now understood, experimental efforts to control the release of fusion energy for net energy output could now be progressed and continue today.
The first fusion experiments were conducted in the Cavendish laboratory in Cambridge, UK, during the 1930’s but results led the eminent scientist Lord Rutherford to pronounce in 1933 that “anyone who looks for a source of power in the transformation of the atom is talking moonshine.”
However after World War II and the technical success of the Manhattan project that developed the first nuclear weapons, an increased interest in atomic physics and fusion in particular was seen.
There was serious interest in the peaceful use of fusion physics all around the world. In fact, in 1951 scientists in Argentina claimed to have controlled the release of nuclear fusion energy. These claims proved to be false but they acted as a spur to many other research groups.
In the UK, much of the early work on fusion was undertaken by universities, principally Sir George Thomson’s group at Imperial College and Peter Thonemann’s team at Oxford, before being centred respectively at Harwell and Aldermaston. Sir George Thomson even developed a patent for a fusion reactor. In 1952 Cousins and Ware built a small toroidal pinch device, but the original large-scale experimental fusion device on which most British fusion physicists worked during the 1940s and 50s was housed in a hangar at Harwell and called the Zero Energy Toroidal Assembly (ZETA). ZETA was a stabilised toroidal pinch device and worked from 1954 until 1958 giving results that showed initial promise and gave clues to later larger devices.
In the US, Lyman Spitzer started the Princeton Plasma Physics Laboratory working on a magnetic confinement device called a stellarator. James Tuck, a British physicist, began work at Los Alamos National Laboratory working on magnetic pinch devices and Edward Teller expanded work on the hydrogen bomb at Lawrence Livermore Laboratory to include inertial confinement techniques.
In the Soviet Union, significant fusion research was also being undertaken. At first all these national projects were shrouded in secrecy, but with the temporary thaw in the Cold War created in 1956 by the visit of the Soviet leaders Nikita Khrushchev and Bulganin to the UK, the first attempts at global co-operation were created. The Russians brought their leading fusion expert academician I V Kurchatov to give a lecture "The Possibility of Producing Thermonuclear Reactions in a Gas Discharge". This described Soviet work in the field and the UK shared its ZETA experience.
Fusion research also started elsewhere (e.g. France and Germany). International co-operation began under the normal scientific exchange of information as countries declassified fusion research, and an Atoms for Peace conference in Geneva in 1958 sealed the start of the process. In the UK this led directly to the setting up of a custom-built laboratory at Culham that would subsequently become the home of the Joint European Torus (JET).
Almost 10 years later (in 1968) results from the Russians Tamm and Sakharov using a new type of magnetic confinement device called a tokamak caused a major stir. Their experiment ran at temperatures ten times higher (10 million degrees centigrade) than anywhere else in the world with excellent confinement results.
The success of the Russians, confirmed by visiting UK scientists in 1969, led to the construction of many tokamak experiments and its position as the dominant technique for fusion research today.
In 1978 the JET project was launched in Europe coming into operation in 1983. The Japanese tokamak JT-60 came online in 1985. In 1991, JET produced for the first time in the world, a significant amount of power (1.7MW or 1.7 million watts) from controlled nuclear fusion reactions. Subsequently, in 1993 the Tokamak Fusion Test Reactor (TFTR) device in Princeton produced 10 MW of power with a plasma fuelled by a 50/50 mix of deuterium and tritium.
In 1997 JET established the current world record for fusion power producing 16 MW of power. All the work in these and other tokamak experiments around the world have given the designers of ITER the information they need to take the next step in the history of fusion.
How does a fusion plant work:
How can fusion produce electricity in a future power plant? The fusion reaction can be simply written as:
Tritium (3H) + deuterium (2H) >> Helium (4He) + a high-energy neutron (n)
In a fusion power plant most of the energy produced by the reactions in the plasma is carried by the neutrons. These high energy neutrons (14 MeV) are captured and their energy used to generate electricity. The energy of the neutrons is absorbed in the structures lining the plasma chamber (known as a torus) walls. The remaining energy in the helium (4He) particles maintains the high plasma temperature. An outline design for a fusion power plant is shown below.
In a fusion power plant the plasma would be confined in a large vacuum vessel surrounded by a neutron absorbing breeding blanket. The breeding blanket has a dual function: it converts the energy of the neutrons into thermal energy and it ‘breeds’ new tritium from lithium to provide more reaction elements.
A large scale vacuum systems is required to ensure an ultra high vacuum in the reactor vessel and to maintain the vacuum surrounding the Superconducting coils located outside the reactor vessel that provide the required strong magnetic field to confine the plasma away from the vessel walls.
Cryogenics (circulating very low temperature liquids) are used to remove the waste and impurities from the plasma, cool the super-conducting magnet coils to allow them to operate, separate the waste gasses into their different individual components for disposal or recycling, provide the cooling for the Radio Frequency heating sources and control the gas pressure of neutral beam systems.
A circulating coolant removes the heat from the blanket and, in the heat exchangers, steam is generated to drive turbines for electricity production.
The main challenge in fusion is to maintain the high temperature of the plasma for long periods of time. In a burning plasma the energy of the Helium nuclei are the main contributors to heating the plasma. However, the plasma is constantly being cooled by impurities picked up from the vessel wall. A divertor system in the vacuum vessel extracts waste gases and power from the plasma and new deuterium and tritium is continuously injected into the plasma.
And the advantage of fusion are:
Almost limitless fuel supply. The basic fuels are distributed widely around the globe. Deuterium is abundant and can be extracted easily from sea water. Lithium, from which tritium can be produced, is a readily available light metal in the Earth’s crust.
No greenhouse gas emissions. Fusion power will not generate gases such as carbon dioxide that are causing growing concern with regard to global warming and other damaging effects on the environment.
Suitable for the large-scale electricity production required for the increasing energy needs of modern cities. A fusion power station will generate a large amount of electricity around the clock.
Waste from fusion will not be a long-term burden on future generations. Only reactor structures close to the fusion plasma will become radioactive. Any radioactive waste generated will be small in volume and the radioactivity will decay over several decades with the possibility of reuse after about 100 years.
The transport of radioactive materials is not required in the day-to-day operation of a fusion power station. The radioactive tritium can be generated and consumed as needed within the reactor.
The system has inherent safety aspects. Only very small amounts of fuel are present in the reactor at any one time. Any malfunction results in a rapid shutdown: ‘runaway’ or ‘meltdown’ accidents are impossible as no chain reaction is involved
. Very low risk of radioactive emissions to the environment. Extensive safety studies have shown that a fusion power station can be operated without significant risk of radioactive emissions. Even in a ‘worst case’ accident scenario there would be no need to evacuate the local population.

link to wiki fusion